1. Field of the Invention
The present invention relates generally to wireless signal processing, and more specifically to circuit techniques for processing signals over antenna arrays.
2. Description of Related Art
The recent proliferation of ultra-wideband (UWB) communication and imaging applications has been widespread in both military and commercial arenas. The utilization of ultra short pulses in time domain, corresponding to ultra wide bandwidth in frequency domain, increases data rate and range resolution in wireless communication and imaging applications, respectively. UWB waves operating at the FCC allocated frequencies propagate through materials with much less attenuation than compared with microwave and optical signals; therefore, UWB imaging systems are ideal for wall and ground penetrating applications and for poor weather conditions. In addition, the power consumption and cost of UWB is usually lower compared to its conventional narrowband counterparts due to UWB's comparatively simpler detection schemes. UWB-based applications include high data rate and secure wireless communications, high resolution radar, biomedical, surgical and environmental imaging, and many others.
The use of multiple antenna transceivers in communications and imaging applications (including UWB) offers spatial diversity and increases communication capacity. In imaging applications using an array of antenna transceivers, a sequence of pulses is transmitted towards and reflects off of the target. The receiver collects the reflected signals and reconstructs the image.
A phased array is a group of narrowband antenna arrays where a sinusoidal plane wave reaches each antenna element with a different phase as a function of incident angle. Variable phase shifters may be used in each path to compensate for the phase offset caused by the propagation delay difference. Depending on the phase shifter settings, the combined received signal of all paths is reinforced in the desired direction and suppressed in the undesired directions.
Timed arrays can be used to enhance the amplitude of a received UWB signal derived from a propagating wavefront. A timed array is a group of antennas in which the relative arrival times at the antennas of the wideband signals vary depending on the propagating wavefront's incident angle. Using delay elements, earlier arriving signals are selectively delayed by variable amounts to produce time-shifted or time compensated signals. The time-shifted signals are added together to form a coherent resulting signal. These procedures occur in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. Timed arrays effectively function as spatial filters, electronically steering the beam towards specific directions to receive signals. Timed arrays can improve receiver signal-to-noise ratios and can also reduce output power requirements.
In conventional architectures, a variable true-time-delay (TTD) element is required for each path of the UWB timed array to compensate for the propagation delay differences. The time delay can be varied by routing the UWB signal through different lengths of transmission lines, which can reside on an integrated circuit chip. FIG. 1 depicts an example of a conventional UWB timed array 100. The timed array 100 includes antenna elements A0-A3, each coupled, respectively, to variable delay elements TTD0-TTD3 and combining element C0. A propagating wave δ is received at antenna components A0-A3 at an incident angle of α as signals δ0-δ3. Here it is assumed that incident angle α results in time delays of magnitude τ, 2τ, and 3τ between the arrival at antenna element A0 of signal element δ0, and the arrivals at antenna elements A1-A3 of received signals δ1-δ3, respectively. The antenna array in this example has a beam array pattern 110 as shown, which is in the same direction as the incident wavefront. The delay elements TTD0-TTD3 each include variable delay paths which introduce different delays into signals δ0-δ3 of 0 through 3τ seconds, respectively, to compensate for the arrival time differences of the four signals. The time-shifted signals δ0TS-δ3TS are summed together in combining element C0 to produce the coherent received signal δC.
While four stages are shown, another number of stages is possible. Further, while a receive technique is demonstrated here, timed array 100 also functions in the same way as a transmitter, with δC instead being the UWB input waveform and C0 a splitter for dividing the signal.
The array of FIG. 1 can alternatively be viewed as a phased array 100 receiving or transmitting a narrowband waveform δ (e.g., a sinusoid). Here, variable phase shifters would be substituted for TTD0-TTD3.
Of particular interest to designers is the development of an integrated UWB timed array. One challenge in the realization of a such an array is the effective implementation of the variable TTD structure. Ideally, the variable TTD should have a small delay resolution in order to achieve high scanning resolution in the array, while having a large total delay to compensate for the delay difference between near and far elements in a large array.
The delay of an electromagnetic wave may be varied by manipulating either the propagation length or the wave velocity. A delay element may use a long delay path, such as an electrical trombone line, to increase the propagation length. However, such an implementation would require an impractically large amount of semiconductor space, as well as consume an excess amount of power. Alternatively, a delay element may rely on changing wave velocity such as by changing the dielectric material. Unfortunately, the transmission line characteristic impedance also varies while modifying the wave velocity, leading to unwanted reflection characteristics at different delay setting.
A similar challenge persists for artisans to develop a more compact, power efficient transceiver circuit for processing narrowband signals. Like the variable TTD elements referenced above, variable phase shifter elements take up space. Conventional approaches require that multiple such elements be used and duplicated to realize existing phased arrays.
In short, the large size and high cost of integrated variable TTD blocks and phase shifters become major issues in conventional narrowband and broadband array architectures.